Preheat oven to 375 degrees. Bake pie as directed on package for 50 minutes. Meanwhile, in a mixing bowl whisk together flour, brown sugar and salt, while breaking up any clumps of sugar as needed. Add butter and cut into mixture using a pastry cutter or fork until it resembles coarse crumbs. Toss in pecans. Chill until ready to use. Once pie has baked 50 minutes remove from oven and sprinkle crumble evenly over pie. Return to oven and bake 20 - 30 minutes longer (tent with foil during the last 10 minutes if needed to prevent excess browning), until center of pie registers 165 - 180 degrees and edges appear set (pie will still giggle slightly, it will set as it cools).
Let cool on baking sheet 10 minutes then transfer pie to a wire rack to cool for 2 hours. Cut into slices. Store pie in refrigerator. Top each serving with whipped topping just before serving.